Turbomolecular pump

ABSTRACT

A turbomolecular pump ( 10 ) includes a rotor ( 18 ) which is mounted on a housing ( 12 ) by at least one roller bearing ( 16 ). The roller bearing ( 16 ) comprises a non-rotating bearing shell ( 20 ) and a rotating bearing shell ( 22 ). The non-rotating bearing shell ( 20 ) is mounted on the housing ( 12 ) by an elastic vibration ring ( 14 ). The vibration ring ( 14 ) is of an anisotropic form, such that the spring stiffness is inhomogeneous over the circumference.

The invention relates to a turbomolecular pump comprising a rotor which is mounted on a housing by means of at least one roller bearing, with the roller bearing being supported on the housing by means of an elastic vibration ring.

Turbomolecular pumps are operated at high rotational speeds of more than 10,000 rpm. Vibrations in the drive assembly of the turbomolecular pump and particularly in the bearings may generate heat, resulting in the possible occurrence of sudden and fast increases of temperature both during permanent operation at a constant rotational speed and during a drop or rise of the rotational speed. Such increases of temperature will cause mechanical stress in the involved component parts and thus will considerably reduce the operating life of the roller bearing and respectively the plurality of roller bearings.

In turbomolecular pumps, it is common practice to support the roller bearings in vibration rings for elastic suspension of the roller bearing within narrow limits relative to the housing. Thereby, it is rendered possible to operate the turbomolecular pump above the second-order rigid-body resonance frequency of the rotor and to keep the bearing forces low when running through the critical rotational speeds as well as during operation at the nominal rotational speed.

As vibration rings, use is made of rings of round section, rectangular rings or rings of other geometric cross sections. Generally, with regard to their cushioning and damping properties, known vibration rings are of a homogeneous nature throughout their circumference. This makes it possible to design a vibratory system wherein the non-rotating bearing shell of the roller bearing will serve as a vibratory mass, and the bearing stiffness and the stiffness of the vibration ring will function as a spring. Thus, the non-rotating bearing shell may happen to perform a movement that is independent from the rotating bearing shell and the housing, thus generating considerable quantities of heat energy.

In view of the above, it is an object of the invention to provide a turbomolecular pump which has an improved vibration behavior.

According to the invention, the above object is achieved by the features defined in claim 1.

The turbomolecular pump of the invention comprises a vibration ring which is anisotropic in the circumferential direction so that the spring stiffness of the vibration ring is not distributed in a uniform manner around its circumference. Thus, the vibration ring does in fact not have a homogenous resiliency and damping effect around its circumference but, instead, a resiliency and damping effect distributed inhomogeneously around its circumference.

By said circumferential inhomogeneity of the vibration ring, build-up of vibration or irregular movement of the non-rotating bearing shell are avoided. Thereby, in turn, a considerable reduction of heat generation is accomplished so that the heat-induced stress of the bearing components, particularly of the non-rotating bearing shell and the rotating bearing shell, will be significantly reduced.

The anisotropic vibration ring and respectively the anisotropy of the ring can be technically realized in various manners.

Preferably, the vibration ring is formed in one piece, and its outer shape is anisotropic along the circumference, i.e. the shape is not invariably constant or regular but is irregular. The vibration ring can comprise e.g. a plurality of radial support sites distributed irregularly along the circumference. In this arrangement, the distance from a support site to its neighboring support site towards one side is unequal to the distance to the neighboring support site on the other side. A support site is to be understood herein as a site on the vibration ring where the non-rotating bearing shell is directly fastened and supported relative to the housing. Of course, also the length of a support site in the circumferential direction can be varied, which holds true also for the mutual distance of the non-supporting regions between the support sites.

Preferably, the vibration ring is formed by an elastic body which preferably consists of an elastomer, wherein said body is continuously closed on the housing side and on the bearing side is interrupted by axial grooves which separate the support sites from each other. There are provided at least three support sites, while, however, preference is given to a number of at least five support sites.

Alternatively or additionally, the vibration ring can be made of different materials distributed along the circumference and having different vibration and damping properties. Such a vibration ring can have an identical cross section along its whole circumference while, however, due to the different materials along the circumference with their different vibration and damping properties, the vibration ring has an inhomogeneous and respectively anisotropic design with regard to these properties.

According to a further alternative embodiment, the vibration ring can also consist e.g. of a helical coil which is made of a uniform spring wire and, depending on the varying pitch, has locally different vibration and damping properties.

An embodiment of the invention will be explained in greater detail hereunder with reference to the drawing.

The FIGURE is a schematic cross-sectional view of a turbomolecular pump comprising a rotor which is supported on the housing by a roller bearing.

Illustrated in the FIGURE is a turbomolecular pump 10 designed for operation at a nominal rotational speed far above 10,000 rpm.

Said turbomolecular pump 10 comprises a housing 12 with a roller bearing 16 held thereon by means of a vibration ring 14, said roller bearing 16 in turn carrying a rotor 18. Roller bearing 16 comprises an outer non-rotating bearing shell 20, an inner rotating bearing shell 22, roller bodies 24 and a roller-body cage 26.

Said vibration ring 14 is formed in one piece of an elastomer and is of an anisotropic form along its circumference. Vibration ring 14 consists of a plastic body 15 which on the housing side is continuously closed along the circumferential direction. Vibration ring 14 comprises eight support sites 30 distributed irregularly along its circumference and formed as support pads facing radially inwardly. Said support sites 30 or support pads 30 are separated from each other by axial grooves 32 of circular cross section. All of said axial grooves 32 have the same width, the same depth and the same shape, but can also be designed differently from each other and particularly have different groove widths, different groove depths and different groove cross-sections.

The groove width n of the axial grooves 32 can—but does not have to—be identical for all axial grooves 32. In contrast thereto, the support site width s varies along the circumference. In any case, the distance a₁ between mutually adjacent support sites 30 towards one side is unequal to the distance a₂ to the adjacent support site on the other side. These distances respectively refer to the center in the circumferential direction of the support sites or pads 30. 

1. A turbomolecular pump comprising a rotor supported on a housing by means of a roller bearing, said roller bearing comprising a non-rotating bearing shell and a rotating bearing shell, and said non-rotating bearing shell being supported on the housing by an elastic vibration ring, wherein said vibration ring is of an anisotropic form such that its spring stiffness is inhomogeneous over its circumference.
 2. The turbomolecular pump according to claim 1, wherein the vibration ring is one-pieced and its form is anisotropic along the circumference.
 3. The turbomolecular pump according to claim 1, wherein the vibration ring comprises a plurality of support sites distributed in a non-uniform manner along the circumference.
 4. The turbomolecular pump according to claim 3, wherein the distance from one of the support sites to a first neighboring support site towards one side is unequal to the distance to a second neighboring support site on the other side.
 5. The turbomolecular pump according to claim 1, wherein the vibration ring is formed by a one-pieced elastic body which on the housing side is continuously closed and on the bearing side is interrupted by axial grooves separating the support sites from each other.
 6. The turbomolecular pump according to claim 1, wherein the vibration ring is made of different materials distributed along the circumference.
 7. The turbomolecular pump according to claim 1, wherein the vibration ring includes a helical coil made of a uniform spring wire.
 8. A turbomolecular pump comprising: a housing; a roller bearing which supports a rotor in the housing, the roller bearing including a non-rotating bearing race and a rotating bearing race; and an elastic ring supporting the non-rotating bearing race in the housing, the elastic ring having anisotropic vibration damping properties with an inhomogeneous spring stiffness over its circumference. 